Field of the Invention
Embodiments relate to removal of nitrate and selenium and other contaminants from wastewater by an integrated process comprising of an electro chemical and biological process, using porous support media as bio-film carrier.
Background of the Related Art
Selenium is widely distributed throughout most soil and natural water. As regulatory limits become more stringent, selenium presents a significant challenge to wastewater treatment. As with any constituent that is present in water at low concentration, it is difficult to consistently achieve low concentration of contaminants in the effluent.
Many different technologies are applied for selenium reduction from wastewater. Very few of them can achieve a desired level of selenium reduction consistently. Each core selenium treatment technology is generally a part of several other treatment technologies or unit processes that form an overall treatment system. This is especially the case when target discharge requirement are less than 10 μg/L. These technologies include physical separation by different membranes, evaporation pond method, chemical treatment, enzymatic reduction, and biological reduction methods, all of which are applied for removal of selenium.
Physical separation methods include membrane filtration and evaporation. Nano-filtration and reverse osmosis are applied for selenium removal. In the case of nano-filtration, pore sizes are similar to the sizes of selenite or selenate oxyanions. Therefore, depending on the exact molecular weight cut off, performance may not be effective.
Reverse osmosis has been demonstrated at full scale to remove selenium. It can remove high levels of total dissolved salt and produce consistent high water quality. Reverse osmosis processes have some disadvantages. They demand high capital cost and operational cost, they have a superior pretreatment requirement to meet membrane tolerance, and they have a higher feed pressure requirement due to higher background total dissolved solids (TDS). Brine concentrators require exotic metallurgy due to high chloride content and become relatively expensive and also generate selenium rich concentrate, which should be either crystallized or sent to evaporation ponds. The main disadvantages of an evaporation pond technique are large space requirements and ineffectiveness in areas of cold climate. Furthermore, evaporation results in a net loss of water and can be concern to areas with scarce of water source. (See “Review of available technologies for the removal of selenium from water,” prepared for North American Metal Council, 2010 Final Report, which is incorporated by reference herein.)
There are three predominant chemical treatment mechanisms by which soluble selenium can be removed from water. Those are precipitation, adsorption, and oxidation/reduction. Adsorption, which utilizes different adsorptive media, is not able to remove selenium to permitted regulatory levels. Other chemical methods like zero-valent iron (ZVI) and catalytic reduction have not been demonstrated to be effective at full-scale plants.
Biological treatment of selenium containing wastewater has been an area of interest, particularly for treatment of high Total Dissolved Solids wastewater. One high TDS wastewater is Flue Gas Desulfurization scrubber blowdown (FGD) wastewater and wastewater generated by the mining industry. In general, biological treatment offers a low-cost alternative to more expensive physical and chemical treatment. Reduction of both nitrate and selenate/selenite takes place at a reduced oxidation-reduction potential (ORP). Biological systems catalyze the reduction of nitrate and selenium. Nitrate gets reduced to inert nitrogen gas while Selenate can be reduced to selenite and both selenate and selenite can be further reduced to elemental selenium. The Nitrogen gas formed gets release in the environment while elemental selenium which is insoluble in water can be removed by solid liquid separation and then disposed of.NO3+organic C→NO2+Organic C→N2+CO2+H2OSeO4+organic C→SeO3+Organic C→Se+CO2+H2O
Oxidation-reduction potential range for the reduction of different oxyanion is shown in Table 1.
TABLE 1Oxidation- Reduction potential rangeApproximateFinal electronORPacceptor(mV)ProcessNitrate/Nitrite   0 to −50De-nitrificationSelenate/Selenit −50 to −200Selenium ReductionSulfate−100 to —Sulfate Reduction
Selenium and nitrate reducing bacteria (FIG. 1) are considered heterotopic; they utilize organic carbon as their electron donor and nitrate, selenate/selenite as their electron acceptors.
To reduce selenium in wastewater, oxygen and nitrate must be reduced. If a significant amount of nitrate is present in water, then a sufficient amount of organic carbon needs to be added to reduce nitrate and selenium. This means that the environment must be controlled to exclude oxygen, and that there must be enough electron donors available to reduce selenium and nitrate to treat wastewater for selenium.
Some examples of carbon sources utilized in reduction of nitrate and selenium include glucose, methanol, acetate, citric acid, molasses, etc. In case of very low concentrations of nitrate and selenium in the wastewater, use of biological treatment processes becomes challenging. Such conditions will result in little or no growth of heterotrophic bacteria, which can lead to washout or potential loss of the microorganism. If electron donors are added in absence of nitrate, selenate, the system could promote sulfate reduction, hydrogen production or even methane generation. Sulfate can also be reduced where selenium gets reduced but sulfate reduction is being carried out by an entirely different group of bacteria. The growth of selenium reducing bacteria is influenced by the pH and temperature of the environment in which bacteria are growing. The best pH range for selenate and selenite reduction is 6.5 to 9.5. Most biological systems for wastewater treatment operate with mesophilic bacteria, or bacteria that operate in the range of 15° C. to 40° C.
Biological processes for selenium removal are widely used. There are certain commercial available technologies that are based on biological processes. (See, e.g., U.S. Pat. No. 6,183,644 and U.S. Pat. No. 7,550,087, both of which are incorporated by reference herein.). Use has been demonstrated in large-scale plants as well. Some of them use attached growth process and other uses suspended growth process. The main drawback of current technologies is that they require high residence time of 4 to 6 hours. They are also highly sensitive to operating conditions and lack of steady ORP control. As the whole biological process depends upon oxidation and reduction potential, it is very important to maintain it even with changing feed water characteristics. Also, in an attached growth process, there is a possibility that the process may get disturbed if sludge gets washed out from the reactor. It is important that the bio-film formed over the media surface remains intact and does not wash out. There is a possibility that bio-culture may wash out from the system during operation or during periodic backwash. If media does not hold the bio-film, it will affect the consistency of the process.